1. Field of the Invention
The present invention relates generally to the fields of radiation dosimetry. More particularly, it concerns the use of scintillating detectors to detect radiation levels in vivo.
2. Description of Related Art
Cancer recurrence is frequently the result of failed local control of treatment options. Reducing the likelihood of recurrence in radiotherapy means giving the largest possible radiation dose to the tumor region while limiting the dose to healthy organs at risk for treatment-induced toxicity. Even with the best treatment planning modalities, there is no guarantee that the prescribed dose will be delivered exactly as planned. Internal movements such as breathing or uncertainties in the filling of organs such as the bladder and rectum may displace the target volume away from the intended treatment field. At the same time, normal tissues may be shifted into the high dose region, introducing unnecessary radiation-induced side effects. The use of in vivo dosimetry to monitor the actual dose received in the target or healthy tissue is the best way to ensure accurate dose delivery. Accurate in vivo dosimetry is an important step towards measuring the dose to the organs at risk in order to assess the quality of the treatment, maximize the dose to the tumor and/or minimize the risk of complications to normal tissues. Presently there are no known available in vivo dosimetry systems that can sample both dose and dose rate fast enough to be suitable for real-time feedback.
With the advent of intensity modulated radiotherapy (IMRT) and conformal proton radiotherapy, there now exists the ability to create a treatment plan with margins small enough to target the tumor while largely sparing the healthy tissues and organs at risk. CT-Linac-based image-guided radiotherapy (IGRT), cone-beam CT and on-board imaging (OBI) systems allows a user to accurately set up a patient relative to the treatment beams just prior to treatment. However, there is no guarantee that the target will remain in the same place during the entire treatment period. Even with the use of increasingly sophisticated radiotherapy techniques, it is difficult to determine if the dose has been delivered exactly as planned. And, if not, it can be difficult to determine the degree to which the actual dose has deviated from the planned dose.
Precise measurement of the dose to organs at risk and other critical structures is therefore necessary to provide a clear picture of the true dose delivered during any given treatment fraction. The benefits of such measurements are multi-fold. First, comparison of the measured dose to that planned will indicate deviations in set up and/or anatomy that indicate the need for changes in the treatment plan. Second, consideration of tumor control and the probability of normal tissue complications will allow a radiation oncologist to either escalate or reduce the dose over the course of a radiotherapy treatment. Finally, knowledge of the true delivered dose will allow for more accurate analysis of organ toxicity. The benefits to the patients will include decreased likelihood of local failure and reduced frequency of life-altering side effects.
The main radiation dosimeters available at the present time include thermoluminescent detectors (TLDs), ionization chambers, radiographic film, silicon (Si) diodes and metal-oxide semiconductor field effect transistor (MOSFET) devices. None of these detectors, aside from Si diodes, allows real-time measurement of dose rate, and none are water equivalent. TLDs, ionization chambers and film do not allow in vivo dose measurements for various reasons (too bulky, slow response, safety concerns, etc.). Even though Si diodes may have good spatial resolution, they suffer from strong energy dependence and are prone to dose perturbation depending on their orientation with respect to the beam (Beddar et al. 1994).
MOSFETs have been commercialized for in vivo dosimetry. Although some systems offer unique advantages including the ability to be permanently implanted and read telemetrically, MOSFETs tend to show angular dependence, energy dependence, and a decreased sensitivity with increasing absorbed dose, requiring either in-house or factory calibration (Soubra et al. 1994; Scarantino et al. 2004; Ramaseshan et al. 2004; Beddar et al. 2005). MOSFET detectors also have a limited lifetime of 70-200 Gy.
Preliminary studies of optically stimulated luminescence (OSL) devices do show promise (Aznar et al. 2004; Aznar et al. 2005). The devices produce spontaneous emission due to radio-luminescence, which exhibits a non-linear dependence with dose rate, as well as optically stimulated luminescence, which can be integrated to obtain the total accumulated dose.
Although these devices can be used for in vivo dose measurements, they suffer from other drawbacks. These include their non-water-equivalence and the required time delay of 5 to 6 minutes needed to retrieve the dose data between measurements. This occurs because the OSL signal related to absorbed dose arises from electron traps that must be optically stimulated with a laser. Thus, one could not use these detectors to discriminate between individual beams in an IMRT or even a two-field proton therapy treatment.
It is clear that none of the detectors described above would satisfy the needs of the real-time in vivo dosimetry that would provide feedback fast enough to enable adaptive radiotherapy. Only plastic scintillators can be used to measure both dose and dose rate in real time. Plastic scintillators have been shown to be water equivalent (Beddar, et. al 1992), linear with dose, dose rate independent, energy independent in the megavoltage energy range, and unaffected by changes in temperature. Moreover the light emission mechanism of a plastic scintillator is fast (nanoseconds) and therefore well suited for real-time applications. So far, several prototypes of plastic scintillation detectors have been proposed for applications in quality assurance, stereotactic radiosurgery, brachytherapy and general dosimetry but no scintillation detector to date can achieve real-time in vivo dosimetry.
A multiple-probe scintillation detector system has been recently developed for quality assurance but can only be used in phantom and not in patients. Moreover the design of this detector system requires an acquisition time that is at least as a long as a radiotherapy treatment fraction, this design flaw prevents any real-time applications.